Perimeter intrusion detection systems using linear detection cables can function based on a variety of physical sensing technologies such as RF leaky cable, guided radar, loose conductor active or passive cables, triboelectric or piezoelectric cables, fiber optic cables, electrostatic fields between conductors, etc. Generally they consist of sensor cables deployed along a line, and a processor to interrogate the cables, either active by sending a signal into the cable and assessing the response, or passive where a signal is received from the cable representing an intrusion. If the response output from the injected signal in active systems is received at the same cable end, and timing relative to the input signal is used, some systems described as “ranging” can perform an additional processing operation to determine the location of the intrusion along the cable linear axis.
The cost viability of such systems is generally assessed on the overall cost per meter of sensor length, which combines the per meter cost of the cables, and the number and cost of processors for the length. The most economical solution generally means large distances between inexpensive processors, and inexpensive cables. However the longer the cables, the more important it is to determine the location of intrusions along the cable. For example, a video camera can be used to assess an alarm and identify the source. This generally means one needs to situate and activate a camera by the sensor with distances less than 100 meters to visually assess the intrusion.
Competitive fence detection sensors can be classified generally into two groups. One group uses relatively complex and costly processing means to provide location information along long cables, and the other, more frequent simpler processors without location means. To be equally cost competitive, in the former case the cables can be more costly than the latter case as the processors are less frequent, while in the latter case both the cables and the processors must be inexpensive.
The perimeter intrusion detection systems are also generally classified either as passive sensing systems, as active sensing systems, or more recently as dual use sensing systems.
Existing cable based linear microphonic sensing systems may work passively, meaning a terminal voltage or charge is produced when the sensor cable is vibrated or deformed by an intruder in proximity. For example, the proprietary Intelli-FLEX™ sensor, sold by Senstar-Stellar, uses a triboelectric effect sensor cable where a small cable terminal voltage is produced when the cable attached to the fence is vibrated, e.g. by an intruder climbing the fence to which the cable is attached. Other sensing means for cables may use the piezoelectric effect, or be based on magnetic materials.
However, there are also fiber optic systems such as IntelliFIBER™ or FiberSensys™, sold by Senstar-Stellar, that are active in that they transmit an optical signal, yet do not provide any location data as they receive a signal modified by a vibration from an intruder, at the opposite cable end. Processing of these signal changes is similar to the passive systems.
Active systems such as the INTREPID MicroPoint™ cable and detection system by Southwest Microwave use a special coaxial cable with loose conductor wires and electronics to detect and locate reflected signal changes from impedance changes produced along the cable. Many of these sensing cables are costly, either because of the materials used or the complexity of processes for construction. For example piezoelectric, triboelectric or electret coaxial cables generally use a special fluoropolymer dielectric material which generates or transfers a charge when flexed or vibrated. These fluoropolymer materials are both costly themselves on a per pound basis, and also more difficult to process compared to many other plastics, as they require high melt temperatures to process, or alternatively may have corrosive properties requiring specialized and costly extrusion equipment. Materials for some cables such as piezoelectric require heating, or stretching for “pre-charging” of the materials. This dielectric material may be extruded but sometimes is best handled by constructing a tape material that is subsequently installed on the cable center conductor by a slow winding process to create the cable dielectric. Electret sensing cables similarly use a production process to heat and charge a fluoropolymer material used for the dielectric. Sometimes the charging is done as a secondary process after the cable is manufactured.
In conventional commercial sensing cables, the piezoelectric cables yield the largest voltage response to a disturbance, with the electret being the weakest, and the triboelectric as an intermediary. Depending on the magnitude of the response relative to ambient noise, there may be a need for additional cable shielding and amplification or other means to improve signal to noise which would adversely affect cost.
Generally, the piezoelectric coaxial cables rely on the continuous contact between the piezoelectric dielectric and the inner and outer conductors for their function, whereas the commercial electret cable create a permanent charge on the dielectric of some polarity, but have an inherent looseness in the braid through its manufacture to allow modulation of a charged capacitor when the cable vibrates. (This is somewhat analogous to, for example, stroking a ferrous material for magnetization in the electret-magnetic arts.) The current commercial triboelectric coaxial cables are distinctive in requiring one of its conductors to be quite loose relative to the dielectric in order to transfer the charge.
Other cables use magnetic materials that again are difficult and costly to process. These magnetic cables require a loose conductor or a plurality of conductors in the dielectric. Even with a relatively complicated manufacturing process they are susceptible to field installation problems due to mishandling.
Fiber optic cables are generally simple in construction; however, they are relatively costly, and have an inherent complexity in processes for installing connectors between cables.
In the prior art literature and based on experiments it would appear that there is little technical distinction between electret and triboelectric principles in creating cables with a terminal voltage in response to a disturbance. The triboelectric series relates to both of these principles in selection of materials for sensitivity, and materials processing prior to manufacturing of cables can be employed to further affect the sensitivity of cables. For example, the same fluoropolymer materials can be used in creating electret, triboelectric, and piezoelectric cables, however it is not clearly understood in the art which component or components creates the voltage. Hence, it is important to have an understanding of how the electret, triboelectric, and piezoelectric charge properties are created.
An electret to create a permanent charge on a material can be formed generally by heating a dielectric close to its melting point, applying a strong electric field and then cooling the dielectric with the field still applied. The result is a residual charge with a lifetime dependent on the material, and it may go through an immediate polarity reversal. Electret processes can also use a corona discharge or an electron beam to produce the charge. However, while there is no clear definition in the art of the best electret materials, except that tetrafluoroethylene (Teflon™) and polyethylene terephtalete (Mylar™) film used commonly in microphone and ultrasonic applications are cited as useful plastics, as are certain ceramics and carnauba wax.
Piezoelectric materials are typically materials such as tetrafluoroethylene as well as other fluoropolymers. The piezoelectric properties are produced by simultaneously heating and applying an electrical field combined with some mechanical stress. Hence, it is possible to have a combination of three (or more) voltage or charge producing factors in some cables which use fluorocarbon dielectrics, for example. However, one distinguishing feature is that piezoelectric cables do not transfer charge through an air gap, but rather are generated by heating and stressing the dielectric. Triboelectric cables require a loose conductor/dielectric interface to transfer a charge.
It is also important to note that the process of creating these electrical charge properties can be prior, during or even accidental. What is meant by this statement is that one can apply heat, mechanical stress, or an electrical field, in a controlled manufacturing process or simply rely on an accidental result occurring in processing.
There are a number of background patents that discuss how to create electrets, which are similar to piezoelectrics as they relate to heating by applying a high direct current (DC) voltage of some polarity, and then providing a cooling process. For example, U.S. Pat. No. 3,316,620, issued to Stewart, describes in detail the process of creating a PVC electret. Other well known methods to create an electret include using a corona discharge. An Internet Encyclopedia on a world wide web site, energy21.org, provides an article, by Geoff Egel, entitled “Electrets vs Dielectric Absorbers” that states: “The list of different ‘plastic’ materials to experiment with is extensive, aside from the usual plexiglas, perspex, acrylic, epoxy, and glasses, [ . . . ]. All of which react differently when charged up, and all of which relate to the triboelectric charging principles—whereby some plastics are charged positive (donator) and some are negative (acceptor).”
Further discussion on the above topic is provided in the following references: “Electrostatics And Its Applications” by A. D. Moore (1973), pp 66; “Handbook of Electrostatic Discharge Controls” by Bernard S. Matisoff (1986), pp 16, “Understanding and Controlling Static Electricity” by G. Luttgens & M. Glor (1989), pp 44, and “Plastics for Electrical Insulation” by Paul F. Burns (1968), pp 50.
Triboelectric cables rely on the use of a combination of materials that are spaced apart on what is known in the art as the triboelectric series to achieve a potential difference. A looseness exists between these materials which then come into contact from a disturbance and frictionally transfer a charge from the triboelectric effect (moving surface interaction). The triboelectric effect is an electrical phenomenon where certain materials is electrically charged or transfers charge when coming into contact with another different material. Based on this electrical phenomenon, the triboelectric cables provide a suitable terminal voltage when flexed or vibrated locally. For example, cables, such as Intelli-FLEX™, have conventionally used tinned copper conductors with a fluoropolymer (FEP), such as Teflon™, as the dielectric material as it is known to have a high response to charge transfer. Unfortunately, using FEP materials mean cable costs are rather expensive when used for large distances based on their cost per meter length. It is also observed that other materials exist as close neighbors in this series and may have acceptable triboelectric properties for this use. These include for example polyvinyl chloride (PVC) and polyethylene.
PVC and polyethylene are common low cost cable materials widely used for cable insulation, most typically polyethylene for coaxial cable dielectrics and jacketing, and PVC for jacketing. Extrusion of these materials is also well known in the cable industry and relies on the simplest process equipment.
Other loose conductor cables may use a number of loosely disposed conductors in special keyways within the dielectric. However, these keyway conductors are relatively complicated to process and are susceptible to field installation problems if mishandled.
U.S. Pat. No. 2,787,784, issued to Meryman et al., on Apr. 2, 1957, discloses a triboelectric sensing device. The sensing device comprises a cable that is physically deformable to provide a triboelectric voltage and an amplifier operatively coupled to the cable to amplify the signal received from the cable in response to a disturbance to the cable. The cable itself consists of two conductive members, and a flexible deformable dielectric conduit loosely spaced between the conductive members. However, the Meryman et al. patent does not disclose the use of a particular dielectric material for the dielectric tubes to enhance the “sensitivity” of the cable, nor does it discuss an active ranging application of the cable. As there is no discussion of specific triboelectric materials, the Meryman et al. patent does not suggest using one material over another in the triboelectric series. The Meryman et al. patent disclosure is limited to a device consisting of a cable and signal amplification means.
In U.S. Pat. No. 3,763,482, issued to Burney et al., on Oct. 2, 1973, the patent discloses an electret coaxial cable for intrusion detection in security systems. This transducer cable comprises an inner conductor and an outer conductor, and a dielectric filler between the conductors where the filler comprises an electret. The cable operates based on the rate of change of position of the outer conductor relative to the cable interior to produce a detectable signal across the inner and outer conductors. Thus, there is an effective air gap between the outer conductor and the outer surface of the filler electret. The air gap creates a discontinuous contact between the outer conductor and the outer surface of the electret, which as a consequence allows the effective air gap to function as a dielectric layer between adjacent faces of the outer conductor and the electret, forming therewith a capacitor. The capacitance level changes as a result of deformation to the cable. A static charge created at the point of deformation is then modulated along the line to produce a detectable signal which is then recorded. The use of materials such as polycarbonate and tetrafluoroethylene (Teflon™) are taught to make the cable highly desirable as they provide a longer charge life than other materials.
Burney et al. further discusses the manufacturing of the co-axial cable with the Teflon™ filler as highly desirable due to its widespread availability. However, Burney et al. does not disclose an active use of the sensing cable. Furthermore, the Burney et al. patent does not disclose a loose center conductor construction. Rather, the looseness is between the dielectric tube and the outer conductor. While the Burney et al. patent does suggest using certain materials due in part to their commercial availability, the selection of filler material is related to their electret properties and as such, materials in the triboelectric series are not specifically discussed in this patent.
U.S. Pat. No. 3,846,780, issued to Gilcher, on Nov. 5, 1974, discloses an intrusion detection system and a cable having an insulated electrical wire loosely positioned within an electrically conductive tube member having an inside diameter substantially greater than the diameter of the wire. This “passive” sensing system utilizes the loose dielectric coated wire in the conductive tube to sense disturbances via the electret effect, or by sensing capacitive changes when there is a DC bias voltage applied to the dielectric coated wire. The Gilcher patent discloses the use of a Teflon™ coated electrical conductor as the preferred dielectric material. Gilcher teaches the use of the clear insulated Teflon™ wire as being better for detecting devices utilizing the electret characteristics of the cable. While PVC was also tested, the material recommended by Gilcher was Teflon™. The Gilcher patent also does not disclose the use of the sensor cable in an active system.
U.S. Pat. No. 4,197,529, issued to Ramstedt et al., on Apr. 8, 1980, discloses a very particular cable configuration comprising an inner metallic conductor centered about the axis of the cable, a thin, substantially flat, horizontal sheet of insulating material, disposed parallel to the horizontal center line which makes contact with and supports the center conductor, and an outer metallic sheath which encloses the conductor and the horizontal sheet. The Ramstedt et al. patent also discloses the cable as part of an intrusion detection system having means for terminating the cable in its characteristic impedance and means connected to the cable for injecting a pulse into the cable which propagates to the end terminating in the characteristic impedance. When an intrusion causes a cable disturbance, the system measures a reflected pulse to determine the location of an intrusion. The Ramstedt patent also discloses that the unique cable design advantageously provides greater sensitivity to motion and vibration. However, this cable construction is rather complex in terms of manufacturing processes and, therefore, not practical cost-wise for applications having larger distances. While the Ramstedt patent discloses an active sensing function, there is no discussion of this particular sensor cable having application for passive sensing by triboelectric effect.
U.S. Pat. No. 5,448,222, issued to Harman, on Aug. 29, 1995, discloses teachings directed towards a transducer cable for detecting the location of a sensed disturbance. According to Harman, the transducer cable contains a center conductor, an outer conductor, a dielectric material between the outer and the center conductor, and a “floating” sense wire conductor located in a slot formed in the dielectric material. While a disturbance of the cable causes the sense wire to move relative to the outer conductor, the sense wire is not constructed as a loosely centered conductor. Rather, Harman primarily teaches a dual slot conductor configuration whereby both the center conductor and the sense wire form a first transmission line and the outer conductor and the sense wire form a second transmission line. A “driving” signal is imposed on the transducer cable in order to obtain a response signal. According to Harman, the location of the intruder is determined from the detected response signal. Harman also teaches a “floating” center conductor, however, the teaching is limited to the center conductor being free to move. While Harman teaches the use of polyethylene as a possible dielectric material, the selection of material to enhance the “sensitivity” of the cable in a passive function is not disclosed. The slot also adds a level of complexity and therefore cost to the manufacture of the cable. Finally, Harman teaches away from triboelectric sensing cables by suggesting their performance is inconsistent from cable to cable.
U.S. Pat. No. 5,705,984, issued to Wilson, on Jan. 6, 1998, discloses an intrusion detection system that provides an active sensing cable whereby multiple simultaneous intrusions may be detected along the cable. Wilson teaches an RF transmission cable that has first and second conductors spaced apart with an insulating material. Wilson further teaches that the cable has a characteristic impedance throughout its length that at any point can change in response to a change in the spacing of the conductors. The cable is buried at a depth that enables the spacing change to occur in response to weight applied proximate to the buried cable. A transmitter provided in the intrusion detection system directs electrical energy into one end of the transmission cable. A portion of that electrical energy is reflected back from any point in the cable that has an impedance that differs from the characteristic impedance. The intrusion detection system utilizes a reflectometer circuit connected to the cable for producing an indication that an intrusion has occurred and the specific location of that intrusion in response to the reflected energy. The system also includes a transfer circuit to separate the transmitted RF energy from the reflected energy. In practice, Wilson teaches that this transfer circuit will be a directional coupler or a similar device known in the art. However, Wilson does not disclose a sensor cable having dual use for both a passive and an active ranging cable system, as the cable construction does not provide a loose center conductor.
There is a need to overcome the shortcomings of the prior art as none of these references disclose nor teach the selection of triboelectric materials, such as polyethylene or other similar material, for enhancing the “sensitivity” of the sensor cable in response to a disturbance or improving their cost effectiveness. Moreover, while several background patents teach various sensing cable constructions, a simple and cost-effective manufacturing process is not contemplated in any of the prior art teachings.
While the Burney '482 patent provides an electret cable that has a loose dielectric similar to triboelectric cables, it requires some electret charging processing in its manufacture. Similarly, the Gilcher '780 patent discloses an electret cable that has a loose conductor but relies on the inherent electret properties of coated wire materials. Manufacturing processes using electrical testing such as dc “hi-pot” testing can deliberately or inadvertently create electret sensitized cables so it is difficult to determine what is the inherent signal level of a specific material created by processing. For example, a strong charge develops on the Intelli-FLEX™ cable after dragging it along the ground. It is possible that extrusion processes, with the plastic still molten or softened, could cause electrification.
In sum, there is a need with existing or planned cables to either optimize or control the processing in manufacture, and later their use, to ensure whatever electrical charge properties are created and maintained for stability purposes.
The stability of the sensor cable response may also vary with the material used and the environmental conditions, i.e., temperature changes, mechanical and electrical stress applied, and humidity level changes. Therefore, the present invention seeks to provide an economic cable that is sufficiently stable within its environment and can be manufactured with low cost materials and simple processes to provide a suitable terminal voltage response to a disturbance.